It is well known to measure torque applied to a rotary or fixed shaft by use of magnetoelastic materials which form part of/is arranged on the shaft since it is known that the permeability of magnetic materials changes due to applied stress. The permeabilities of the magnetoelastic materials change due to the fact that a magnetoelastic material changes its magnetic properties when it is subject to tensile and compressive stresses respectively. Therefore, the torque and/or axial stresses to which an object is exposed must either be transmitted to tensile and compressive stresses of the magnetoelastic material which is fastened onto the object or the object itself (or part of it) comprises a magnetoelastic material.
JP-A-166 827/1984 shows a non-contacting torque transducer wherein thin magnetoelastic ribbons are attached to the surface of a shaft where said ribbons form a herring-bone pattern, two pick-up-coils and an excitation coil being used as detecting means. The excitation coil is excited with a frequency of 20 kHz.
Another known non-contacting torque sensor is based on a three-layer principle. The innermost layer, i.e. closest to the shaft comprises a highly permeable non-magnetostrictive amorphous layer whereafter a layer comprising a non-magnetic material follows. The outermost layer comprises a high permeable magnetostrictive amorphous material. An excitation frequency of about 100 kHz is used. More particularly the sensor comprises three layers in order to reduce the sensitivity to stray fields.
In EP-A-0 422 702 a magnetoelastic torque transducer is disclosed which uses an excitation frequency of 1-100 kHz and preferably a frequency of 10-30 kHz i.e. just above the human audible range in order to avoid whistling.
Also in US-A-4 823 620 a magnetostrictive device for measuring torsional torque is shown which likewise use an excitation frequency of 1-100 KHz.
All known torque sensors of this kind suffer the drawback of being sensitive to external magnetic stray fields. This severely limitates the use of such sensors, amongst others might even the earth magnetic field influence the signal. When using excitation frequencies of a few tenths of kHz it might be necessary to use advanced shielding devices. An object with the present invention is to provide a sensor having a high sensitivity, is easy to handle and manufacture and has a fast response time. It should further be insensitive to stray fields and withstand various kinds of e.g. dirty or greasy environments. Furthermore it should be usable with a simple coil system i.e. not require a very complicated coil system or similar.
Further problems reside in the fact that the sensors are strongly dependent on temperature. A certain reduction in drift due to temperature can be achieved through carefully keeping the layer of glue or similar by which the magnetoelastic material is applied to the object at an identical and uniform thickness all over the area where it should be used (different thicknesses of the glue layers e.g. under different ribbon belts causes the two output signals to drift differently so that when one signal is subtracted from the other, the final output will drift). Furthermore a certain reduction in temperature dependence can be achieved through fixing the coil bobbin in order to restrict its movements depending on temperature changes. Due to the fact that coil bobbins normally are made of materials which per se have a high coefficient of thermal expansion the coils are displaced when the temperature changes. For a number of applications however, said measures do not imply that the temperature dependence of this sensor is reduced to an acceptable level.